std::basic_ios::operator bool

From cppreference.com


operator void*() const;	(1)	(until C++11)

explicit operator bool() const;	(2)	(since C++11)

1) Returns a null pointer if fail() returns true, otherwise returns a non-null pointer. This pointer is implicitly convertible to bool and may be used in boolean context.

2) Returns true if the stream has no errors occurred and is ready of I/O operations. Specifically, returns !fail().

This operator makes it possible to use streams and functions that return references to streams as loop conditions, resulting in the idiomatic C++ input loops such as while(stream >> value) {...} or while(getline(stream, string)){...}. Such loops execute the loop's body only if the input operation succeeded.

#include <iostream>
#include <sstream>
 
int main()
{
    std::istringstream s("1 2 3 error");
    int n;
    std::cout << std::boolalpha << "(bool)s is " << (bool)s << '\n';
    while (s >> n) {
        std::cout << n << '\n';
    }
    std::cout << std::boolalpha << "(bool)s is " << (bool)s << '\n';
}

Output:

(bool)s is true
1
2
3
(bool)s is false

[edit] See also

`ios_base::iostate` flags			basic_ios accessors
eofbit	failbit	badbit	`good()`	`fail()`	`bad()`	`eof()`	`operator bool()`	`operator!()`
false	false	false	true	false	false	false	true	false
false	false	true	false	true	true	false	false	true
false	true	false	false	true	false	false	false	true
false	true	true	false	true	true	false	false	true
true	false	false	false	false	false	true	true	false
true	false	true	false	true	true	true	false	true
true	true	false	false	true	false	true	false	true
true	true	true	false	true	true	true	false	true

Language
Standard library headers
Concepts
Utilities library
Strings library
Containers library
Algorithms library
Iterators library
Numerics library
Input/output library
Localizations library
Regular expressions library (C++11)
Atomic operations library (C++11)
Thread support library (C++11)

std::basic_ios::operator bool

Contents

[edit] Parameters

[edit] Return value

[edit] Example

[edit] See also